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Hatching time and alevin growth prior to the onset of exogenous feeding in farmed, wild and hybrid Norwegian Atlantic salmon.

ABSTRACT: The onset of exogenous feeding, when juveniles emerge from the gravel, is a critical event for salmonids where early emergence and large size provide a competitive advantage in the wild. Studying 131 farmed, hybrid and wild Norwegian Atlantic salmon families, originating from four wild populations and two commercial strains, we investigated whether approximately 10 generations of selection for faster growth has also resulted in increased somatic growth prior to the onset of exogenous feeding. In addition, we tested whether relaxed selection in farms has allowed for alterations in hatching time between farmed and wild salmon. Across three cohorts, wild salmon families hatched earlier than farmed salmon families, while hybrid families displayed intermediate hatching times. While the observed differences were small, i.e., 1-15 degree-days (0-3 days, as water temperatures were c. 5-6°C), these data suggest additive genetic variation for hatching time. Alevin length prior to exogenous feeding was positively related to egg size. After removal of egg size effects, no systematic differences in alevin length were observed between the wild and farmed salmon families. While these results indicate additive genetic variation for egg development timing, and wild salmon families consistently hatched earlier than farmed salmon families, these differences were so small they are unlikely to significantly influence early life history competition of farmed and wild salmon in the natural environment. This is especially the case given that the timing of spawning among females can vary by several weeks in some rivers. The general lack of difference in size between farmed and wild alevins, strongly suggest that the documented differences in somatic growth rate between wild and farmed Norwegian Atlantic salmon under hatchery conditions are first detectable after the onset of exogenous feeding.

SUBMITTER: Solberg MF

PROVIDER: S-EPMC4249964 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Increased knowledge of heritable traits in Atlantic Salmon (Salmo salar) is important to overcome bottlenecks in salmonid aquaculture. Atlantic salmonid populations, both landlocked and anadromous, represent an interesting model to gain insight into anadromy related traits, most notably, the probability to smoltify. While a previous study has identified several genomic regions diverging between anadromous and landlocked populations across the species range, the present study explores these data further with the aim to uncover if some of these genomic regions are linked to beneficial genetic traits associated with smoltification. In this study 17 of these loci were monitored in 669 anadromous salmon originating from 36 full-sibling families that had been reared under common garden conditions. The Smolt Index was calculated, using multiple visual markers, and provided a means of assessing smoltification stage. One SNP, located in Ssa04, showed a significant association with probability to smoltify, where individuals homozygous for the landlocked variant (LL) displayed a decrease in probability of smoltifying after one winter when compared with the homozygous for the anadromous variant (AA). This effect was independent of individual fish size. A separate common garden study comprising 200 individuals from either anadromous or landlocked strains showed that expression levels of ncor1, a thyroid mediator hormone located on the same chromosomal region (Ssa04), were significantly reduced in landlocked individuals post smoltification but remained constant in their anadromous counterparts. This study therefore suggests that while size is still the most important trigger for the induction of smoltification, there may also be an additional genetic component or trigger that has been 'lost' during the years deprived of SW transfer. In conclusion, the LL genotype identified here could potentially be used by the industry to delay smoltification and may also represent one of the first clues to the genetic regulation of smoltification in Atlantic salmon.

Project description:BackgroundDomestication of Atlantic salmon for commercial aquaculture has resulted in farmed salmon displaying substantially higher growth rates than wild salmon under farming conditions. In contrast, growth differences between farmed and wild salmon are much smaller when compared in the wild. The mechanisms underlying this contrast between environments remain largely unknown. It is possible that farmed salmon have adapted to the high-energy pellets developed specifically for aquaculture, contributing to inflated growth differences when fed on this diet. We studied growth and survival of 15 families of farmed, wild and F1 hybrid salmon fed three contrasting diets under hatchery conditions; a commercial salmon pellet diet, a commercial carp pellet diet, and a mixed natural diet consisting of preserved invertebrates commonly found in Norwegian rivers.ResultsFor all groups, despite equal numbers of calories presented by all diets, overall growth reductions as high 68 and 83%, relative to the salmon diet was observed in the carp and natural diet treatments, respectively. Farmed salmon outgrew hybrid (intermediate) and wild salmon in all treatments. The relative growth difference between wild and farmed fish was highest in the carp diet (1: 2.1), intermediate in the salmon diet (1:1.9) and lowest in the natural diet (1:1.6). However, this trend was non-significant, and all groups displayed similar growth reaction norms and plasticity towards differing diets across the treatments.ConclusionsNo indication of genetic-based adaptation to the form or nutritional content of commercial salmon diets was detected in the farmed salmon. Therefore, we conclude that diet alone, at least in the absence of other environmental stressors, is not the primary cause for the large contrast in growth differences between farmed and wild salmon in the hatchery and wild. Additionally, we conclude that genetically-increased appetite is likely to be the primary reason why farmed salmon display higher growth rates than wild salmon when fed ad lib rations under hatchery conditions. Our results contribute towards an understanding of the potential genetic changes that have occurred in farmed salmon in response to domestication, and the potential mechanisms underpinning genetic and ecological interactions between farmed escapees and wild salmonids.

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Hatching time and alevin growth prior to the onset of exogenous feeding in farmed, wild and hybrid Norwegian Atlantic salmon.

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